Growth of native tree species planted in montane reforestation projects in the Colombian and Ecuadorian Andes differs among site and species

The tropical Andes in Ecuador and Colombia are a biodiversity hotspot that has endured centuries of forest clearance and degradation. Forest restoration has been identified as a regional conservation priority; in recent decades, native species reforestation projects have proliferated, but little information exists on growth performance of commonly planted tree species in relation to site and soil nutrient status. This study analyzed growth of seven common native species (Alnus acuminata, Baccharis bogotensis, Cedrela montana, Myrica pubesens, Quercus humboltii, Sambucus nigra, Smallanthus pyramidalis) on 12 montane forest sites across the northern region of the tropical Andes. Andean alder (A. acuminata) was the most commonly planted species, and grows at a mean annual diameter increment (MAI-d) of 1.81 cm y(-1) and a mean annual height increment (MAI-h) of 0.95 m y(-1). S. pyramidalis, a short lived pioneer of the Asteraceae family, also exhibited fast growth rates of 1.64 cm MAI-d and 1.21 m MAI-h. Andean oak (Q. humboltii) was the second-most commonly planted species, growing with an MAI-d of 0.99 cm and MAI-h of 0.56 m. Soil magnesium and potassium were significant predictors of MAI-d and MAI-h for A. acuminata, while soil nitrogen, phosphorous, sodium, and calcium were negatively associated with growth (p < .001). We speculate that A. acuminata did not grow as well on soils richer in calcium and phosphorus because they were less conducive to nitrogen symbiosis common to this species. Soil magnesium and calcium were significant predictors (p < .05) of diameter growth for Q. humboltii. For both species, we attribute growth responses to soil nutrients as a result of the variable nature of fertility in the complex and variable soils that make up the volcanic and surficial geological landscape of the northern Andes. Results indicate that native species can grow in a variety of soil conditions, and exhibit growth rates comparable to non-native species. However, our results suggest native species are site restricted for best growth and should be planted on particular soils. We make recommendations for reforestation for the species in this study.